An audio amplifier mainly uses a class AB amplifier and a class D amplifier. The efficiency of each amplifier is illustrated in FIG. 1.
FIG. 1 is an exemplary diagram of the efficiency of general class AB amplifier and class D amplifier.
As illustrated in FIG. 1, the class D amplifier has an output efficiency significantly higher than that of the class AB amplifier. A general class A or AB amplifier has an efficiency of approximately 20% to 50%. The loss of efficiency results in heat at an amplifier output terminal, and a heat sink plate is required for cooling. As a consequence, the class A or AB amplifier has high power loss, and since it is heavy and large, it is not portable.
The class D amplifier has high output efficiency at high output levels, but has low output efficiency at low output levels. In reality, the audio amplifier mainly operates at low output level rather than maximum output level. Therefore, a technology for maintaining high output efficiency at low output levels is very required. In this regard, the class D amplifier varies power to realize high output efficiency, regardless of audio output.
FIG. 2 is an exemplary diagram of the efficiency of the general class D amplifier according to a power supply voltage.
The audio amplifier varies power to maintain a high efficiency. As illustrated in FIG. 2, in the case where a power supply voltage is a power supply voltage 1 (H1), a power supply voltage 2 (H2), a power supply voltage 3 (H3), and a power supply voltage 4 (H4), when audio output is constant, the audio amplifier may have high output efficiency as the power supply voltage is increased.
However, when the power is varied, noise is generated in the audio output signal that is output from the audio amplifier. The maximum output of the audio amplifier is obtained by dividing the square of the power supply voltage of the audio amplifier by the resistance value of a load, as expressed by Equation 1 below. When the power supply voltage of the audio amplifier is instantaneously varied, the output of the audio signal varies and noise is generated. When the efficiency of the audio amplifier is assumed to be 100%, the audio output may be calculated using Equation 1 below.0=(V×Vin/Vmax in)2/R  Equation 1
In Equation 1 above, 0 denotes audio output, V denotes an audio supply voltage, Vin denotes audio input, Vmax in denotes a maximum audio input value, and R denotes a load resistance.
As expressed by Equation 1 above, when the audio supply voltage is varied, the audio output also varies. At this time, when the audio supply voltage is rapidly varied, noise is generated.